The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling line pressure of the transmission.
A known continuously variable belt-drive transmission (Japanese Patent Laid Open No. 55-65755) for a motor vehicle comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to servo device, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices. The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine.
In a conventional electronic control system (Japanese Patent Laid Open No 60-73160), a drive pulley speed sensor, a driven pulley speed sensor, an engine speed sensor and a throttle position sensor are provided. Actual transmission ratio (i) is calculated by a calculator based on the output signals (N.sub.P) of the drive pulley speed sensor and the output signal (N.sub.S) of the driven pulley speed sensor with a formula (i=N.sub.P /N.sub.S). Desired transmission ratio (id) is calculated based on the output signal (N.sub.S) of the driven pulley speed sensor and the output signal (.theta.) of the throttle position sensor. The actual transmission ratio i is controlled to the desired transmission ratio id by shifting the spool of the transmission ratio control valve.
On the other hand, engine torque T.sub.e is calculated based on the output signal .theta. of the throttle position sensor and an output signal Ne of the engine speed sensor. Necessary line pressure P.sub.LU is decided by the actual transmission ratio i. Further, desired line pressure P.sub.L is decided by the engine torque Te and necessary line pressure P.sub.LU (P.sub.L =P.sub.LU .times.Te). However, the output of the engine is transmitted through a clutch. Accordingly, the line pressure should be obtained in accordance with the clutch transmitting torque Tc (P.sub.L =P.sub.LU .times.Tc). In such a system, in slipping conditions of the clutch at the start of the vehicle, the line pressure decided by the engine torque becomes higher than an actually necessary line pressure, which causes disadvantages such as losses of operations of the pump and pulley, reduction of starting characteristic of the vehicle, and others.